No, it is not illegal to use profane language to a minor, without more. (It may be a violation of Facebook's terms of service, but that is not a crime.)
In general, if you'll forgive my going a bit further than was asked, there is no good reason to remain 'friends' with a hostile ex-partner on social networking sites. People today seem to have this weird idea that Facebook is the arbiter and forum for all social relationships. It's really not. Facebook is a privately-owned town square. Everything you do there is public and permanent, and it's not required for communication. If you can't deal with your ex-girlfriend with equanimity - if you don't want to see regular public reports from her - it's perfectly proper to remove her from your list. Certainly more proper than cursing at her - there's no way that can accomplish anything good.
For more thoughts on this, I recommend Dan Savage's advice - especially this sort of thing: http://www.youtube.com/watch?v=csCozsjPrhg
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It is not illegal, but could be a violation of Facebook's rules.
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